Restrictive and non-restrictive relative clauses

A restrictive relative clause (defining relative clause) is that clause that explains the noun preceding it. It provides the essential information and its absence makes the sense of the expression incomplete.

In British English, that/which introduce restrictive relative clauses when talking about things, but not humans. Both that and which are interchangeable.

Note: You don’t need a comma in front of a restrictive clause.

non-restrictive (non-restrictive) relative clause
A non-restrictive relative clause (non-defining relative clause) is an extra information that can be left out of an expression without changing its meaning. It uses which, whom, whose, who, but never that.

Basketball is a good sport, which I like personally.

They talked to the manager, who really had all they needed.

Note: You need a comma to separate a non-restrictive clause from the main clause.

When you use non-restrictive relative clause in the middle of a sentence, use a comma before and after it.

Dan, the captain of the team, sat with the coach at the press briefing.

The coat, which I wore yesterday, was cheaper at the mall.

more explanation

If you’re still confused about how to identify what restrictive and non-restrictive relative clauses are, think about it this way:

A restrictive clause is ‘restricted’ because the complete sense of the expression is confined within it. A non-restrictive clause is the opposite—the meaning is not confined within it, so it can be left out. This is why we use comma to set it apart.